1. Field of the Invention
The present invention relates to the production of methyl tertiary butyl ether (MTBE) from the reaction of isobutylene (iC.sub.4.sup.=) with methanol (MeOH). More particularly the invention relates to a process where high purity iC.sub.4.sup.= may be used as the feed while still maintaining adequate temperature control and iC.sub.4.sup.= selectivity to MTBE. Most particularly the invention relates to a catalytic distillation process wherein an inert C.sub.4 hydrocarbon is initially fed to a distillation column reactor to provide a heat sink and dilute the reactants. After start up the initial feed of the inert C.sub.4 hydrocarbon is ceased and that fed is retained in the system.
2. Related Art
The production of MTBE from the acid catalyzed reaction of iC.sub.4.sup.= and MeOH is well known in the art. Generally the iC.sub.4.sup.= is contained in a mixed hydrocarbon stream containing predominantly C.sub.4 's which includes normal butenes, butanes and possibly lighter C.sub.3 hydrocarbons. The iC.sub.4.sup.= content of these streams is typically from 10-70 mole %. The MeOH preferentially reacts with the iC.sub.4.sup.= to form MTBE with the remainder of the materials in the mixed hydrocarbon passing through essentially as inerts.
One major difficulty with the iC.sub.4.sup.= /MeOH reaction has been temperature control due to the exothermicity of the reaction. Several methods of temperature control have been applied including indirect heat exchange in the catalyst bed, inter-bed cooling and quench. One method which has found wide spread acceptance is catalytic distillation wherein the heat of reaction simply causes boil up of the material in the catalyst bed. The temperature is controlled by the pressure. This particular method is exemplified by commonly owned U.S. Pat. Nos. 4,232,177; 4,307,254; and 4,336,407. A variation utilizing vaporization of the mixture for heat removal is disclosed in Canadian Pat. No. 929,537 wherein the vaporized portion is condensed and returned to the reactor, there being no distillation or separation. Additionally U.S. Pat. No. 4,540,831 discloses substantially the same process as the Canadian reference wherein all of the overheads are condensed and both products and unreacted materials are withdrawn as bottoms.
The catalytic distillation method of reaction works well when there is sufficient material within the bed to act as a heat sink-that is, there is sufficient material within the bed to absorb all of the heat of reaction without complete vaporization in the bed. After complete vaporization, the heat would simply be added as sensible heat and increase the temperature.
In U.S. Pat. No. 4,540,831 the broad embodiment of the process comprises exothermally reacting a first chemical compound and second chemical compound in a reaction zone to form a third chemical compound, vaporizing the first or second compound to remove heat and condensing the vapor overhead and removing the third compound and substantially all of the unreacted first and second compound in the bottoms effluent steam. The patent for example describes an MTBE process using as a feed an admixture of C.sub.4 hydrocarbons including butanes and isobutylene in a process where the MTBE formed within the catalyst bed and the remaining C.sub.4 hydrocarbons descend through the catalyst bed and are removed as a single combined effluent stream.
The use of concentrated iC.sub.4.sup.= as a feed stock for MTBE processes presents special problems because of the heat of reaction and the potential loss of selectivity due especially to dimerization. A simple solution to this problem would be to dilute the iC.sub.4.sup.= feed with inerts that boil in the reaction temperature range as the more common feed streams already are. The best diluents would therefore be other C.sub.4 's, such as the butanes and normal butenes in the mixed hydrocarbon streams available.
However, these other C.sub.4 's have value as feedstocks to other processes and while they are not appreciably consumed in the MTBE process, they do become contaminated, especially with MeOH and other oxygenated products which reduce their value as feedstocks as, for example HF alkylation. More significantly the dilution of a substantially pure isobutylene feed with sufficient inert diluents, e.g., 10 to 70% isobutane based on isobutylene, results in the processing of large quantities of materials to separate them from the product. Thus, by dilution, a pure reactant feed is contaminated to dampen the reaction and removed to get the product, which requires larger equipment.